Adsorption Of Small Organic Molecules Research Articles

DFT screening of adsorption of biodiesel molecules on aluminum and stainless steel surfaces

We present a Density Functional Theory (DFT) study of the adsorption of small organic molecules, taken as models of biodiesel autoxidation products, on stainless steel and aluminum surfaces. The anhydrous and fully wetted (0001) α-Al2O3 and α-Cr2O3 surfaces were taken as models for aluminum and stainless steel surfaces, respectively. Only on the anhydrous surfaces, a clear ranking in adsorption energy is observed: carboxylic acid > methyl esters > alcohol ∼ketone > aromatics > alkanes. The different surfaces reactivity range in the order anhydrous Al2O3 > anhydrous Cr2O3 >∼hydroxylated Al2O3 and Cr2O3. Calculated desorption temperatures show that only on aluminum-based surfaces some of the investigated organic molecules remain adsorbed at operational conditions (500 K).

Design of 3D Carbon Nanotube Monoliths for Potential-Controlled Adsorption

The design of 3D monoliths provides a promising opportunity to scale the unique properties of singular carbon nanotubes to a macroscopic level. However, the synthesis of carbon nanotube monoliths is often characterized by complex procedures and additives impairing the later macroscopic properties. Here, we present a simple and efficient synthesis protocol leading to the formation of free-standing, stable, and highly conductive 3D carbon nanotube monoliths for later application in potential-controlled adsorption in aqueous systems. We synthesized monoliths displaying high tensile strength, excellent conductivity (up to 140 S m−1), and a large specific surface area (up to 177 m2 g−1). The resulting monoliths were studied as novel electrode materials for the reversible electrosorption of maleic acid. The process principle was investigated using chronoamperometry and cyclic voltammetry in a two-electrode setup. A stable electrochemical behavior was observed, and the synthesized monoliths displayed capacitive and faradaic current responses. At moderate applied overpotentials (± 500 mV vs. open circuit potential), the monolithic electrodes showed a high loading capacity (~20 µmol g−1) and reversible potential-triggered release of the analyte. Our results demonstrate that carbon nanotube monoliths can be used as novel electrode material to control the adsorption of small organic molecules onto charged surfaces.

Selective adsorption of toluene-3,4-dithiol on Si(553)-Au surfaces

The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered.

The effect of solvation and temperature on the adsorption of small organic molecules on calcite.

We performed density functional theory calculations to investigate the effect of solvation and temperature on the adsorption of small organic molecules on calcite. The Conductor like Screening Model for Real Solvents (COSMO-RS) solvation model was used to describe a multicomponent mixture consisting of both hydrophobic and hydrophilic phases. The results demonstrate that the combination of solvation and temperature significantly influences adsorption, with the effect of temperature dominating over the effect of solvation. At 25 °C, carboxylic acids and methanol are stable on calcite with free energy of adsorption <0 in the hydrophobic phase. None of the molecules considered in this study remain on the surface in the hydrophilic phase.

SMALL MOLECULES ADSORBED ON SILICON SURFACES: TWO CASES THROUGH TIME

The adsorption of small organic molecules on silicon surfaces has been long a subject of investigations, as it provides the fundamental basis of silicon-based technologies in many fields. Several approaches were used, both theoretical and experimental, on many types of adsorbate-substrate systems aiming at determining preferential sites and geometries of adsorption, stable configurations, transition barriers, adsorption mechanisms, electronic structures among others. The research efforts, though, did not always bring to conclusive arguments and on some systems investigations are still going on following the evolution of the experimental techniques and computational methods. In this review, two case studies are reported: benzene and methanol on Si(100)2[Formula: see text]1, i.e. examples of a molecular and a dissociative adsorption. The adsorption of benzene on Si(100)2[Formula: see text]1 is still an open case, as it may adsorb in di-[Formula: see text] or tetra-[Formula: see text] bonded configurations, but contrasting evidences have been reported so far, on which of the two is the most stable one and the debate is still open. The adsorption of methanol is less controversial and it is widely accepted it is dissociative with breakage of the O–H at low coverages. But also in this case, investigations are going on to elucidate the adsorption mechanism.

Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite.

Understanding strength and nature of noncovalent binding to surfaces imposes significant challenge both for computations and experiments. We explored the adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas chromatography and theoretical calculations, providing new insights into the strength and nature of adsorption of small organic molecules on these surfaces. The measured adsorption enthalpies on fluorographite range from −7 to −13 kcal/mol and are by 1–2 kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity of organic adsorbates to fluorographene than to graphene. The dispersion-corrected functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86b-vdW) achieved the best agreement with the experimental data. Computations show that the adsorption enthalpies are controlled by the interaction energy, which is dominated by London dispersion forces (∼70%). The calculations also show that bonding to structural features, like edges and steps, as well as defects does not significantly increase the adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.

Ionic complexation and adsorption of small organic molecules on calcium silicate hydrate: Relation with their retarding effect on the hydration of C3S

The ion complexing power and the adsorption of set-retarders such as sugar derivatives (d-glucitol, d-gluconate…), amino-carboxylates and phosphonates (EDTA, EDTMP, …) on C-S-H were identified and compared. The complexation equilibria of the predominant calcium and/or hydroxide complex and its relative constant were determined after measuring the solubilizing effect of the molecules in suspensions buffered by portlandite. The adsorption of the molecules on C-S-H as well as their possible complexation with silicate ions in solution were also investigated. All the molecules studied are shown to complex calcium and/or hydroxide ions but only amino-phosphonates, d-gluconate and d-glucitol complex silicate ions. They all adsorb on portlandite and C-S-H but to different extents. These organic compounds are also revealed to be sensitive to the presence of calcium counter-ions at the surface of C-S-H. Finally, the relation between their ionic complexing powers, adsorption properties and retarding effects on the hydration of C3S are discussed.

Asphaltene adsorption on quartz sand in the presence of pre-adsorbed water

Background and hypothesisIn the oil industry, asphaltenes are known for their tendency to aggregate in solution and to deposit on surfaces, with both properties being connected with operational problems associated with recovery, production and refining. Interactions involving asphaltenes and other crude oil components have been a major aspect of investigation in attempting to understand the full nature of these problems. Water is implicated in asphaltene behavior in solution, for example, where it has been found to delay the deposition of asphaltene aggregates. At interfaces, there is evidence that water-in-crude oil emulsion stability is enhanced through asphaltene-water interactions, and there have been a few reports that asphaltene adsorption on mineral surfaces is reduced in the presence of water. We consider the latter aspect, because the instances of reduced adsorption to date have not attempted to quantify the effect. Previous studies showed that vapor-phase adsorption of organic molecules is reduced in the presence of pre-adsorbed water and we were interested to determine whether the same is true for liquid-phase adsorption of asphaltenes. ExperimentsThe surface of quartz sand was controlled by pre-adsorption of water from different relative humidity (RH) environments as the water adsorption isotherm is known from previous studies. These pre-conditioned sand samples were used as substrates for the adsorption from toluene solutions of n-heptane-precipitated asphaltenes (C7A) from an Athabasca oil sands bitumen. To supplement the adsorption behavior, atomic force microscopy (AFM) images were taken of the resultant sand grains at 0 and 80% RH, and low-field NMR was used to estimate the sand wettability. FindingsAsphaltene adsorption on sand is sensitive to surface-adsorbed water, with ∼4-fold reduction in adsorption when increasing the RH from 0 to 100%. This is in general agreement with previous vapor-phase adsorption of small organic molecules, and is therefore believed to be the first demonstration of the effect of pre-adsorbed water on adsorption from solution. Asphaltene adsorption as a function of RH is the converse of literature water adsorption isotherm data. Three asphaltene adsorption regions have been tentatively identified based on water adsorption behavior and the literature interpretation of water structure on quartz: the highest asphaltene adsorption occurs at very low RH, decreasing to a near-constant value in the range ∼40–80% RH, followed by a rapid decrease beyond 80% RH. Further analysis strongly suggests that asphaltene adsorption decreases linearly with the thickness of the adsorbed water film. The effects of RH on asphaltene adsorption are also reflected in AFM images and NMR wettability results.

Realistic molecular cluster models for exfoliated kaolinite

AbstractMolecular cluster models, developed for an exfoliated kaolinite, provide a structural description comparable to that of periodic slab models for a fraction of the computational cost. These models include both the octahedral and the tetrahedral sheets of kaolinite. The first-generation model (G1) contains the inner and outer coordination sphere of the Al- and Si-honeycombs as the preferred sites for adsorption of small organic molecules. Since no experimental information is available to date at the atomic level for exfoliated kaolinite, we carried out a systematic density functional theory evaluation for establishing the most reasonable coordinates of the ions and groups. The results of molecular cluster and periodic calculations were utilized for evaluating semi-empirical Hamiltonians on larger models. Using a PM7 Hamiltonian, the structure of cluster models containing 1 + 6 (second generation) and 1 + 6 + 12 (third generation) Al- and Si-honeycombs which are out of reach for ab initio calculations, were determined. These molecular slab models offer a structural platform for adsorption, intercalation and delamination studies.

Theoretical study of conformational disorder and selective adsorption of small organic molecules in the flexible metal-organic framework material MIL-53-Fe

Many of the potential applications for metal-organic framework (MOF) materials require molecular level understanding of their adsorption of small organic molecules, which are not readily accessible from experiment. Through high-level van der Waals corrected, hybrid density functional theory calculations, we elucidate the adsorption configurations of several, representative small organic guest molecules in an archetypal flexible MOF material, MIL-53-Fe. The predicted relative energies between low-energy adsorption configurations of 1,4-benzoquinone in MIL-53-Fe are in very good agreement with the thermal transition temperatures observed experimentally and suggest that thermodynamic factors govern the precise arrangements and loading of guests in the MOF host. Experimentally observed conformational disorder of small organic molecules in MIL-53-Fe is explained by predicted multiple low-energy adsorption configurations that are comparable with the thermal energy of the guests, kT. Finally, we show that the previously observed selective adsorption of pyridine and 2,6-lutidine molecules in water by MIL-53-Fe, can be rationalised through a careful analysis of the host–guest and guest–guest interactions and is controlled by thermodynamic factors.

Insight into the Effect of Long Range Interactions for the Adsorption of Benzene on Transition Metal (110) Surfaces

In recent years, evaluating the effect of van der Waals (vdW) forces for many physical systems, including the adsorption of small organic molecules on metal surfaces, became possible thanks to the continuous improvements in vdW density functional theory (DFT). In this work, employing the vdW-DF (revPBE-vdW), the optimized vdW-DF (optB88-vdW, optPBE-vdW, optB86b-vdW), and vdW-DF2 (rPW86-vdW2) methods, we study the adsorption of benzene on a range of (110) transition metal surfaces. We evaluate the performance of the vdW functionals by examining the equilibrium adsorption geometries and energies and comparing the results with the available experimental data and the PBE calculations. Our results show that the optimized vdW-DF functionals predict the equilibrium geometries in good quantitative agreement with some of the available experimental reports. We show the crucial importance of the vdW interactions for accurate description of equilibrium geometries and energies of benzene on transition metal (110) surf...

Simple model to study heterogeneous electrocatalysts

New electrocatalyst materials have been proposed to increase the performance of fuel cells. Experimental studies show that Pt and Pb metallic and oxide materials are quite efficient in the oxidation of alcohols and small organic molecules such as formic acid in advanced fuel cells. This work proposes a model for studying morphologically heterogeneous catalysts through quantum chemistry methods such as density functional calculations. For testing the model, we have experimentally studied the adsorption of small organic molecules, namely formic acid and methanol, on Pt and Pb electrodes. All methodologies we have tested can be employed for this kind of study, but M06 functional results correlate best with previous simulations of homogeneous catalysts and with experimental data obtained for homogeneous and heterogeneous electrodes. Our model indicates that the presence of a Pt–Pb interface is responsible for higher adsorption energies of these molecules, most likely due to the orientation of the organic molecules that should facilitate the oxidation process.

Atomic-scale inversion of spin polarization at an organic-antiferromagnetic interface

Using first-principles calculations, we show that the magnetic properties of a two-dimensional antiferromagnetic transition-metal surface are modified on the atomic scale by the adsorption of small organic molecules. We consider benzene (C6H6), cyclooctatetraene (C8H8) and a small transition metal - benzene complex (BzV) adsorbed on a single atomic layer of Mn deposited on the W(110) surface -- a surface which exhibits a nearly antiferromagnetic alignment of the magnetic moments in adjacent Mn rows. Due to the spin-dependent hybridization of the molecular pz orbitals with the d states of the Mn monolayer there is a significant reduction of the magnetic moments in the Mn film. Furthermore, the spin-polarization at this organic-antiferromagnetic interface is found to be modulated on the atomic scale, both enhanced and inverted, as a result of the molecular adsorption. We show that this effect can be resolved by spin-polarized scanning tunneling microscopy (SP-STM). Our simulated SP-STM images display a spatially-dependent spin-resolved vacuum charge density above an adsorbed molecule -- i.e., different regions above the molecule sustain different signs of spin polarization. While states with s and p symmetry dominate the vacuum charge density in the vicinity of the Fermi energy for the clean magnetic surface, we demonstrate that after a molecule is adsorbed those d-states, which are normally suppressed due to their symmetry, can play a crucial role in the vacuum due to their interaction with the molecular orbitals. We also model the effect of small deviations from perfect antiferromagnetic ordering, induced by the slight canting of magnetic moments due to the spin spiral ground state of Mn/W(110).

Adsorption of Small Organic Molecules on Graphene

We present a combined experimental and theoretical quantification of the adsorption enthalpies of seven organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate, hexane, and toluene) on graphene. Adsorption enthalpies were measured by inverse gas chromatography and ranged from -5.9 kcal/mol for dichloromethane to -13.5 kcal/mol for toluene. The strength of interaction between graphene and the organic molecules was estimated by density functional theory (PBE, B97D, M06-2X, and optB88-vdW), wave function theory (MP2, SCS(MI)-MP2, MP2.5, MP2.X, and CCSD(T)), and empirical calculations (OPLS-AA) using two graphene models: coronene and infinite graphene. Symmetry-adapted perturbation theory calculations indicated that the interactions were governed by London dispersive forces (amounting to ∼60% of attractive interactions), even for the polar molecules. The results also showed that the adsorption enthalpies were largely controlled by the interaction energy. Adsorption enthalpies obtained from ab initio molecular dynamics employing non-local optB88-vdW functional were in excellent agreement with the experimental data, indicating that the functional can cover physical phenomena behind adsorption of organic molecules on graphene sufficiently well.

Analysis of the origin of lateral interactions in the adsorption of small organic molecules on oxide surfaces

A decomposition scheme is proposed to analyze the physical contributions to the decrease in the binding energy of chemisorbed species with increasing coverage. This scheme is applied to the acetaldehyde–TiO2 (110) rutile system as a model for other small organic molecule—oxide surface systems. Different density functional theory (DFT) functionals have been employed at both low-medium and high coverages to understand how the different theoretical descriptions of the various terms influence the adsorbate–surface interaction. At low coverages, it is found that the localized adsorbate to surface electron donation is the fundamental physical process that influences the adsorbate–surface interaction. This results shows that while it is usually assumed that only pairwise adsorbate–adsorbate interactions influence the adsorption energy, the progressive modification of the surface properties (surface reduction in this case) may also play a significative role. The DFT+U functional results, in this case, in the best agreement with the experimental binding energy, and the inclusion of the dispersive forces results in largely overestimated adsorption energies. At higher coverages, the pure GGA and GGA+U functionals overestimate the repulsive terms and the computed binding energy is well below the experimental data. The inclusion of the dispersive forces is required to correctly reproduce the experimental results. The contributions of the different physical terms are also analyzed.

Adsorption of small organic molecules on anatase and rutile surfaces: a theoretical study

The adsorption of several small organic molecules on rutile (110) and (100) as well as on anatase (101) surfaces was investigated by Car-Parrinello molecular dynamics in aqueous solution and a new approach to the calculation of adsorption energies is proposed, taking into account the potential energy fluctuation of larger systems. Acetylene and ethylene insert into twin oxygen vacancies in the surface and form polarized covalent Ti-C bonds. In one case spontaneous coupling of two acetylene molecules to a C(4)H(3) molecule with a structure similar to trans-butadiene was observed. Neutral catechol and the singly charged anion were not reactive on any titanium dioxide surface, but the twofold-charged anion attained stable mono- and bidentated geometries on anatase. Methanol, ethanol, formaldehyde and acetaldehyde adsorbed with their functional groups. Very stable geometries provide a Ti-O bond and have adsorption energies of 60-200 kJ/mol. The adsorbates compete with water molecules for similar adsorption sites in point defects as well as on perfect surfaces.

Interaction of Organic Molecules with the TiO2 (110) Surface: Ab Inito Calculations and Classical Force Fields

We have studied the adsorption of a number of organic molecules consisting of methyl, benzyl, and carboxylic groups on the rutile TiO2 (110) surface using both ab initio and atomistic simulation techniques. We have tested the applicability of a simple embedded cluster model to studying the adsorption of small organic molecules on the perfect rutile TiO2 (110) surface, and used this model to develop a classical force field for the interactions of a wide class of organic molecules consisting of these groups with the rutile TiO2 (110) surface. The force field accounts for physisorption and ionic bonding of organic molecules at the surface. It allows the reproduction of adsorption energies and of geometries of organic molecules on the rutile surface. It should be useful for studying diffusion of these molecules and their manipulation with use of AFM and STM tips.

Pore distribution effect of activated carbon in adsorbing organic micropollutants from natural water

Adsorption isotherms of organic micropollutants in coexistence with natural organic matter (NOM) were analyzed to evaluate the impacts of pore size distribution of activated carbon (AC) on the competition effects of the NOM. Single solute adsorption experiments and simultaneous adsorption experiments with NOM contained in a coagulation-pretreated surface water were performed for four agricultural chemicals and three coal-based activated carbons (ACs) having different pore distributions. The results showed that, for all the carbons used, the adsorption capacity of the chemicals was reduced distinctly in the presence of NOM. Such a reduction was more apparent for AC with a larger portion of small pores suitable for the adsorption of small organic molecules and for the agricultural chemicals with a more hydrophilic nature. Ideal adsorbed solution theory (IAST) incorporated with the Freundlich isotherm expression (IAST-Freundlich model) could not interpret the impact of NOM on the adsorption capacity of the chemicals unless a pore blockage effect caused by the adsorption of NOM was also considered. By taking into account this effect, the adsorption isotherm of the chemicals in the presence of NOM was well described, and the capacity reduction caused by the NOM was quantitatively assessed from the viewpoints of the site competition and the pore blockage. Analytical results clearly indicated that pore blockage was an important competition mechanism that contributed to 10–99% of the total capacity reductions of the chemicals, the level depended greatly on the ACs, the chemicals and the equilibrium concentrations, and could possibly be alleviated by broadening the pore size distributions of the ACs to provide a large volume percentage for pores with sizes above 30 Å.

Guest selective adsorption from the gas phase onto a functional self-assembled monolayer immobilized on a super-sensitive quartz-crystal microbalance

A self-assembled monolayer of alkanethiols having functional groups ( HS-(CH 2) 10− X; X = −H, −COOH, −CONH 2 ) was immobilized on Au electrodes of a quartz-crystal microbalance (QCM), and the adsorption behavior of gaseous organic molecules, such as aliphatic acids, amines, alcohols, and alkanes, was observed from frequency decreases (mass increases) of the QCM plate in gas-phase. We used a super-sensitive, seventh overtone frequency of a 9 MHz AT-cut QCM (63 MHz) in order to detect a monolayer adsorption of small organic molecules. Association constant ( K a ), adsorption ( K 1), and desorption rate constants ( K −1) could be obtained from curve fitting of time courses of frequency changes (mass changes). All organic molecules hardly adsorbed onto the alkane ( X = − H) membrane. To the -CONH 2 membrane, only acetic acid can be selectively adsorbed compared with the -H membrane, but not caproic and nonanoic acids, which explains that hydrogen-bond interactions are important but hydrophobic interactions are not. To the -COOH membrane, both acetic acid and ethyle-nediamine can largely adsorb compared with the -H membrane, but not monoamines. The adsorption of ethylenediamine can be explained by acid-base interactions.

On the formation of organic coatings on marine particles: Interactions of organic matter at hydrous alumina/seawater interfaces

The interaction mechanism between aquatic organic matter (AOM) and mineral particles in seawater has been studied using hydrous alumina as a model mineral phase. The electrochemical methodology allowed direct measurements in oxide suspensions and distinction between ‘dissolved’ and ‘particulate’ organic matter. Adsorption of small organic molecules, such as salicylic acid, was found to be negligible at natural concentration levels. Unsaturated lipids are the most reactive surface-active constituents of AOM leading to multilayer formation. For humic materials an inverse relationship has been established between absorbed densities and particle concentration and it is interpreted as the fractionation effect of a polydisperse solute. A new, highly surface-active fraction of AOM has been identified as fluid surface-active aggregates, ubiquitous in productive surface waters, sea surface microlayers and in mixing zones in estuaries. Hydrophobic attraction is the most important force governing AOM—particle interaction in seawater. Reversible adsorption of organic molecules at charged marine interfaces is only the initial step in the process of organic coating formation that involves complex structural organization phenomena well known in membrane sciences and biophysics.